Ultra-high purity ammonium hydroxide is commonly used in the manufacture of semiconductor products such as microprocessors. In particular this is used as a cleaning solution in the formation of integrated circuits. As these circuits become smaller and smaller, impurities become less tolerable. In particular, the ammonium hydroxide utilized must be free of any conducting contaminants and in particular metallic contaminants. There is also an integrated circuits market for high purity specialty gas ammonia.
Generally commercially produced ammonia is totally unsuitable for such applications. The production of ammonia such as for use in fertilizers introduces contaminants including oil and metal particles. This commercial grade or fertilizer grade ammonia may include up to 10 ppm free oil and several ppm cadmium, calcium, sodium, iron, zinc and potassium. To be useful for integrated circuit production the metal concentration should be less than about 100 ppt.
There are a number of processes which are designed to purify this commercial grade ammonia. However for various reasons, they are not optimally designed. Hoffman et al., U.S. Pat. Nos. 5,496,778 and 5,846,386 disclose drawing ammonia vapor from a liquid ammonia reservoir and passing the vapor through a filter capable of filtering out particles. Due to the amount of impurities in the ammonia, removing these impurities in the vapor phase is inefficient and to a large extent ineffective. The small size of many of the metal particles makes vapor filtration ineffective. Further the evaporator design permits entrainment of the impurities.
Japanese Patent 8-119626-A discloses passing ammonia gas through a saturated aqueous solution of ammonia. This allows entrainment in the mist and requires a subsequent mist separator. Further due to the design of the device, particles are not given sufficient time to be removed in the liquid bath. There are filtration processes designed to remove oil from ammonia. But these are not capable of to producing ultra high purity ammonia.
The present invention is premised on the realization that ultra-pure ammonia can be obtained from commercial grade ammonia by first filtering/coalescing liquid ammonia with a liquid phase filter/coalescer to remove almost all of the oil and metal particles which are carried by the oil. The partially cleaned ammonia liquid is then directed to a quiescent evaporator which promotes vapor formation without creating turbulence or bubbles which would promote entrainment of impurities. After vapor phase filtration, the ammonia vapor can be further directed to an aqueous scrubber. The aqueous scrubber is particularly designed to provide small bubbles which follow a path through a saturated water bath. The path provides sufficient time for any particles within the bubbles to contact the bubble wall and migrate into the aqueous liquid. Further, this is done with minimum turbulence to prevent again entrainment of impurities. The ammonia vapor collected from the scrubber can be combined with ultra-high purity water to form an ultra-high purity solution of ammonium hydroxide suitable for use in the semi-conductor industry. If ammonia gas on anhydrous ammonia is the desired product, purified ammonia gas for this purpose can be collected after vapor phase filtration.
This process can be practiced in a large free standing plant or can be scaled down to provide on-site production of ammonium hydroxide or ammonia gas.
The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings in which: